1. Field of the Invention
The present invention is directed to a method, in the form of a pulse sequence, for operating a magnetic resonance (MR) tomography apparatus, as well as to a magnetic resonance tomography apparatus operating according to the pulse sequence method.
2. Description of the Prior Art
A so-called xe2x80x9csteady statexe2x80x9d pulse sequence that is usually referred to with the acronym xe2x80x9cFISPxe2x80x9d is disclosed in detail, for example, in U.S. Pat. No. 4,769,603. This pulse sequence is distinguished by a high spatial resolution, a short measuring time (in the seconds range per slice) and by a high signal-to-noise ratio for tissue having fluid consistency. This is particularly true of the sequence version referred to as xe2x80x9cTRUE FISPxe2x80x9d wherein the gradients in all directions are reset after each read out phase before the next-successive excitation pulse. The signal of the FISP sequence is essentially determined by the ratio T1/T2 of the relaxation times T1 and T2. The nuclear magnetic resonance signal becomes higher as this ratio becomes smaller. Given a high excitation angle xcex1 of, for example, 90xc2x0, the nuclear magnetic resonance signals derive from the initial magnetization M0 according to the following equation:
S≈M0/(1+T1/T2)
This signal, however, occurs only after many excitation cycles dependent on the excitation angle xcex1 and on the ratio T1/T2. Soft tissue such as, for example, muscle with T1=600 ms and T2=50 ms has a relatively high ratio T1/T2=12 and therefore appears with relatively low intensity in the FISP image. In contrast thereto, fat has a relatively low T1/T2 ratio of approximately 270 ms:70 ms=3.85. In the steady state, fat therefore yields a very high signal intensity. This is particularly disturbing because fat is typically not the subject of the examination.
As already mentioned, the aforementioned signal intensity is only valid in the dynamic equilibrium condition, i.e. only after many excitation cycles. Until then, the signal intensity undergoes pronounced oscillations that last for a major part of the measuring time without further measures. In this time span, the nuclear magnetic resonance signals cannot be used for the imaging since they would lead to pronounced artifacts in the phase-coding direction during the transient event.
The oscillations of the magnetization, and thus of the nuclear magnetic resonance signal, can be largely avoided when, as disclosed in U.S. Pat. No. 5,541,514, the pulse sequence is preceded by a radio frequency pulse in the form of an xcex1/2 pulse, i.e. a radio frequency excitation pulse having half the flip angle of the following radio-frequency excitation pulses. The nuclear magnetic resonance signals that arise thus can be used for the imaging from the very start. In the conventional type of phase coding that proceeds from the highest negative value through zero to the highest positive value, however, this does not change anything with respect to the high signal intensity of spectral components having a low T1/T2 ratio. The image contrast is mainly determined by the middle (central) k-space rows, as is known, for example, from the article by W. Nitz: xe2x80x9cBildgebende Sequenzen in der Kemspintomographie und ihre klinische Anwendungxe2x80x9d, which appeared in electromedica 64 (1996), No. 1, pages 23 through 29. Since data in these rows are only acquired in the middle of the overall measurement, i.e. after many preceding excitations, the steady state condition having a high signal intensity of fat has already been reached.
The article by David A. Feinberg et al., xe2x80x9cHalving MR Imaging Time by Conjugation: Demonstration at 3, 5 kGxe2x80x9d, Radiology 1986, Vol. 161, pages 527-531, discloses a method for reducing the image data acquisition. To that end, only one half of the k-space is filled with raw image data acquired from the subject; the other half of the k-space is filled with synthetic data that, due to their symmetry, can be determined from the measured data.
An object of the present invention is to modify a pulse sequence of the FISP type such that the high signal intensity of undesired spectral components is avoided and to provide an MR apparatus that can be operated with such a pulse sequence.
These objects are inventively achieved in an MR pulse sequence and in an MR apparatus wherein the central area of k-space are already measured shortly after the beginning of the sequence when an equilibrium condition of the magnetization has not yet occurred. In this condition before reaching equilibrium, nearly all diagnostically relevant tissue exhibits a higher magnetization than in the equilibrium condition, whereas the magnetization of undesired spectral components, for example fat-bound protons, still oscillates and exhibits no significant super-elevations compared to the steady state.
In the inventive pulse sequence and in the MR tomography apparatus operated according to the sequence, in a first pulse sequence section, a radio frequency pulse with a flip angle of xcex1/2 is emitted into an examination subject. In second and third pulse sequence sections, a sequence of radio frequency pulses each having a flip angle xcex1 is emitted, with a repetition time that is lower than the T1 and T2 relaxation times of the examination subject. The third pulse sequence section begins before the steady state equilibrium condition in the subject is reached. In the third pulse sequence section, magnetic resonance signals are read out with phase coding after each radio frequency pulse, with the phase coding being reset before the next-following radio frequency pulse is emitted. The magnetic resonance signals acquired in this manner are entered into a k-space matrix sorted according to phase factors. The phase coding ensues such that in the third pulse sequence section, the central rows of the k matrix are acquired at the beginning of the third pulse sequence section.
Advantageously, the amplitude of the phase-coding gradient rises linearly in the third pulse sequence section. When the direction of the phase-coding gradient thereby changes from repetition to repetition, data are entered proceeding from the center of the k-space toward the edges of the k-space in both directions. Alternatively, data for only half the k-space can be obtained since this fundamentally contains all of the information required for the image reconstruction.
In an embodiment, the number of radio frequency pulses in the second pulse sequence section is selected such that the signal amplitude of nuclear magnetic resonance signals of unwanted spectral components has a minimum at the beginning of the third sequence section. This embodiment utilizes the fact that the magnetization of the undesired spectral components still oscillates in this region. When the beginning of the measurement, i.e. the measurement of the k-space center, is timed such that the magnetization of the unwanted spectral components happens to exhibit a minimum, this spectral component is optimally suppressed in the image.